Bacterial methionine N-terminal peptidase

ABSTRACT

Methods for obtaining N-terminal methionine-free proteins involve a novel E. coli methionine amino peptidase. The method is capable of in vitro or in vivo application. For in vivo application, a plasmid-borne DNA encoding the peptide is expressed in a bacterial host.

This is a continuation-in-part of U.S. Ser. No. 778,414, filed Sept. 20, 1985.

TECHNICAL FIELD

The invention relates to production of proteins, especially foreign proteins, lacking an N-terminal methionine, in bacterial systems using recombinant techniques. More specifically, it relates to a peptidase specific for N-terminal methionine which can be used in vitro or used to create a superior bacterial host containing elevated levels of peptidase for complete processing of mature proteins in these systems.

BACKGROUND ART

Production of foreign proteins in bacterial hosts is now well established. In relatively standard procedures, the gene sequence encoding the desired protein is placed under the control of regulatory sequences indigenous to or compatible with the host and transformed into the host bacterium. In general, there are three major ways in which this can be accomplished: (1) the DNA encoding the desired protein can be fused in reading frame with a bacterial gene already under the control of the bacterial regulatory sequences to obtain a "fusion protein"; (2) the desired coding sequences can be fused in reading frame with an operable leader sequence resulting in a secreted protein; or (3) the desired coding sequence is placed immediately downstream from an ATG start codon which results in "direct" expression to obtain the "mature" protein. In this last instance, the mature protein is not secreted but is found in an intracellular location, often as a refractile or inclusion body.

It is often left unstated, but well recognized by practitioners in the art, that the mature protein formed by the direct expression of ATG-preceded coding sequences frequently bears an N-terminal methionine which is the translation product of the ATG. Depending on the particular recombinant protein, and on the circumstances of its production, more or less of it may be processed in the cell to remove this N-terminal Met residue, but, in general, at least some, and usually a substantial portion of the recombinant protein produced does bear this unwanted foreign residue. Its presence is not completely harmless. When the resulting proteins are used therapeutically, what would ordinarily be an autologous protein to the recipient (for example, human growth hormone (hGH) as administered to humans) now contains a peptide sequence which is unfamiliar to the recipient. The result is predicatable. An immune response may be mounted to the unfamiliar sequence and a therapeutically important peptide now becomes an immunogen.

In addition to foreign proteins, mature native proteins and bacterial proteins from other genera or species are also often incompletely processed. Examples of this phenomenon include E. coli aspartate transcarbamylase (R-chain), E. coli tryptophan synthestase A protein, and E. coli bacteriophage T4 lysozyme. (Fasman, G. O., Ed., CRC Handbook of Biochemistry & Molecular Biology, III:308-313.)

Others have attempted to resolve the N-terminal methionine problem and to produce N-terminal "Met-less" peptides or proteins in various ways. Baxter (U.S. Pat. No. 4,350,764) uses in vitro treatment with the protease trypsin to cleave a precursor protein after protecting alternate cleavage sites. Fusion proteins have also been synthesized where the desired coding sequence is preceded by ATG, thus providing an "internal" methionine in the fusion cleavable by CNBr. Not only does this involve an extra preparation step, but has the more serious defect that the protein or peptide is also cleaved at any methionine residues in the remaining sequence. EPO publication 127,305, published Dec. 5, 1984, to Genentech discloses the production of Met-less hGH by employing a coding sequence which includes the native hGH leader peptide which apparently is workable in certain bacterial hosts to effect secretion of the resulting hGH. Gilbert (U.S. Pat. No. 4,338,397) has employed the penicillinase leader sequence to effect the secretion of presumably N-terminal Met-less β-globin. U.S. Ser. No. 715,653, filed Mar. 25, 1985, assigned to the same assignee and incorporated herein by reference, discloses the use of the leader sequence for bacterial phospholipase A (phoA) to effect secretion of certain, but not all, recombinant peptides.

None of the foregoing approaches provides a universal solution to the problem. Faced with the necessity to produce any particular recombinant protein in N-terminal Met-less form, the practitioner needs to select from a repertoire of possibilities a procedure suitable for the particular peptide to be produced. The method of the invention described below expands this repertoire to provide still another pattern of applicability.

DISCLOSURE OF THE INVENTION

The invention furnishes a convenient enzyme for assuring the processing of N-terminal methionine residues from bacterially produced recombinant or other proteins. Also provided is the DNA encoding this enzyme permitting genetic manipulation of the recombinant host organism to effect the desired processing in vivo without the necessity of the separate step. The availability of the peptidase enzyme of the invention thus permits production in bacterial hosts of recombinant proteins which have reduced immunogenicity when used therapeutically.

In one aspect, the invention relates to a peptidase which cleaves N-terminal methonine residues from certain protein sequences--i.e., a methionine amino peptidase or N-terminal exopeptidase. The enzyme will be referred to herein as Met-aminopeptidase. The Met-aminopeptidase enzyme includes proteins which have activity of defined specificity (see below) and which immunoprecipitate with antibodies prepared against the protein of amino acid sequence shown in FIG. 2 herein. The Met-aminopeptidase enzyme further includes proteins having this activity and which are encoded by DNA that hybridizes under specified conditions to the DNA illustrated in FIG. 2 encoding the deduced amino acid sequence. The invention also relates to antibodies immunoreactive with protein of the amino acid sequence shown in FIG. 2, including those raised by injection of this protein into a vertebrate.

In other aspects, the invention relates to a recombinant DNA sequence encoding the Met-aminopeptidase, to plasmids bearing this sequence, and to microbial hosts transformed with these plamids, as well as to methods for producing N-terminal Met-less recombinant proteins in bacterial hosts or in vitro using the materials of the invention. In still another aspect, the invention relates to a method to obtain a bacterial strain with high Met-aminopeptidase activity comprising screening generally peptidase-deficient (but not Met-aminopeptidase deficient) transformant hosts wherein the transformant has had introduced a portion of its own genome.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the N-terminal amino acid sequence determined from the protein of the Met-aminopeptidase of the invention.

FIG. 2 shows 1.2 kb insert of pSYC1174 encoding the Met-aminopeptidase illustrated in FIG. 1.

FIG. 3 shows an SDS gel of purified recombinant IL-2 before and after processing with the enzyme of the invention.

FIG. 4 shows the gene for ricin A.

MODES OF CARRYING OUT THE INVENTION A. Definitions

As used herein, "Met-aminopeptidase" refers to an enzyme which specifically cleaves the N-terminal methionine residue from a peptide sequence, and does not cleave at internal methionine residues or at N-terminal residues other than methionine. The Met-aminopeptidase of the invention appears to be specific for particular peptide sequences depending on the residue occupying position 2 and on the secondary or tertiary structure of the substrate protein or peptide. The precise specificity of the enzyme in this regard cannot be determined without testing nearly an infinite number of substrates; however, a useful rule of thumb is set forth by Sherman, F., et al, Bio Essays (1985) 3:27-31. Sherman et al based their considerations for specificity of Met-aminopeptidases on the observed forms of mutants of iso-1-cytochrome-C from yeast and the published primary sequence of 82 mature intracellular proteins. They conclude that methionine is usually cleaved from residues with a side chain having a radius of gyration of 1.29 Å or less, but generally not cleaved from residues with side chains larger than 1.43 Å. This is consistent with the observation that mutationally altered iso-1-cytochrome-C taken in consideration with other published sequences of other proteins from procaryotic and eucaryotic systems indicate that N-terminal methionine is cleaved when it precedes residues of alanine, cysteine, glycine, proline, serine, threonine, or valine, but not when it precedes residues of arginine, asparagine, aspartic acid, glutamine, glutamic acid, isoleucine, leucine, lysine, or methionine. These results are generally consistent with those set forth in the illustrations below. However, some exceptions occur where the radius of gyration for the second amino acid is higher than 1.29 Å and the secondary and tertiary structure or other conditions are particularly favorable. Therefore, this aspect of the specificity is intended as a general guideline and it should be borne in mind that even the specificity of the aminopeptidase used to illustrate the present invention is not determined with exact precision, i.e., not all possible tertiary structures have been tested. Therefore, in order to fall within the definition of "Met-aminopeptidase", the enzyme needs only to meet the requirement of specific cleavage of N-terminal methionine without cleavage of internal methionine residues and without cleavage of N-terminal residues other than methionine from any peptide.

The preferred Met-aminopeptidases of the invention have N-terminal amino acid sequences substantially equivalent to that shown in FIG. 1, and total amino acid sequences substantially equivalent to that encoded by the DNA illustrated in FIG. 2. By "substantially equivalent" is meant that the protein retains the same activity in specifically cleaving N-terminal methionine residues from particular peptide or protein sequences with fundamentally the same underlying specificity with respect to secondary and tertiary structures or subsequent amino acid residues, even though minor alterations in amino acid sequence may be present. Such alteration, interchange, addition, or deletion of one or several amino acids in the sequence which do not appreciably alter activity do not remove a particular protein from this definition.

For example, conservative amino acid substitutions, such as substitution of a serine or an alanine residue for one or more of the cysteines at positions 45, 59, 78, 126, or 245, result in peptides which may retain the Met-aminopeptidase activity. In addition, there may be interchangeability of leucine, isoleucine, and valine residues. Also, up to the first seven amino acids at the N-terminus may be deleted, and portions of the downstream regions of the peptide beginning at amino acid 228 may not be essential for activity.

Of course, included as well are the neutral and salt forms of the Met-aminopeptidases, as well as forms which contain additional non-protein moieties such as glycosylation, lipid residues, or acetylation.

"Peptidase-deficient" strain refers to a bacterial strain which is lacking in at least four peptidases other than Met-aminopeptidase which peptidases are normally found in the wild type.

"N-terminal Met-less" protein, as used in this application, specifically refers to a protein lacking an N-terminal methionine, but which might include methionine residues elsewhere in its primary structure. The coding sequence for the protein will have an immediately preceding ATG start codon in reading frame. "N-terminal Met-less" is used as a convenient shorthand term so that the conditions surrounding this state need not be repetitively given. Specifically, "N-terminal Met-less" does not mean, in the context of this invention, that there are necessarily no methionine residues whatsoever in the protein sequence, nor is it intended to include proteins which had no possibility of an N-terminal methionine in the first place, such as those produced as fusion proteins or as secreted proteins, preceded before processing by a signal sequence. Thus, as used herein, "N-terminal Met-less protein" refers to a protein which is encoded by a DNA sequence wherein the mature protein is immediately preceded by an in-reading-frame ATG start codon and not part of a fusion to be subsequently cleaved by CNBr, trypsin, or other reagents. In the case of recombinant proteins, the constructions are clearly specified; for native or naturally recombined DNA sequences, the constructions may not be as easily defined.

"Met-preceded protein" is a protein which contains an N-terminal methionine residue immediately preceding the first amino acid normally found in the particular mature protein.

"Cells", "cell cultures", "host cells", and the like refer to subject cells for recombinant DNA manipulations. As would be apparent from the context, these cells may be candidates for, or resultants of, transfer of new DNA sequences according to recombinant techniques. Techniques which are suitable for DNA uptake by cells include most prominantly, in vitro transformation. However other techniques such as transduction or conjugation may also be used. The definition further includes the progeny of the cells directly referred to. It is understood that such progeny may not be precisely identical in DNA content to their parents, but such progeny are included in the definition so long as alterations due, for example, to accidental or deliberate mutation do not destroy the ability of the cells to exhibit the properties conferred by the introduced DNA in a manner similar to that exhibited by their parents.

B. General Description

The present invention achieves the production of N-terminal Met-less recombinant proteins in bacterial hosts through the use of a Met-aminopeptidase. In the most preferred embodiment, the Met-aminopeptidase is generated at high levels in situ and processes the recombinant or other protein in vivo in the bacterial host. However, it is also possible to isolate or extract the desired recombinant or other protein from the host cells and treat the extract in vitro with the Met-aminopeptidase obtained independently from a bacterial source.

With respect to the Met-aminopeptidase per se, this enzyme has been produced and the DNA encoding it recovered by taking advantage of the ease in screening E. coli strains which are generally peptidase deficient for enhanced production of the Met-aminopeptidase encoded in their own genome. In this process, a source of plasmid-borne and amplified DNA encoding Met-aminopeptidase is obtained, and the enzyme can then be prepared and purified directly from these cells. In addition co-transformation of recombinant hosts with this plasmid DNA, along with the expression vector for a desired recombinant protein, results in in situ production of the recombinant protein in N-terminal Met-less form.

A number of E. coli strains which are deficient in a multiplicity of peptidases ordinarily found in wild-type bacteria are known. For example, Miller, C. G., et al, J Bacteriol (1978) 135:603-611, discloses a number of bacterial strains which are peptidase deficient. One of these strains, CM89, which is deficient in peptidase N, peptidase A, peptidase B, peptidase D, and peptidase Q, was used in the illustration below. However, other peptidase-deficient mutants of bacterial strains could be used as well.

Presumably, the genome in these strains still contains the sequence encoding the Met-aminopeptidase activity, and the genomic DNA is therefore used to construct a library by digestion with an appropriate restriction enzyme and cloning the fragments into carrier vectors. A variety of such carrier vectors is available, including the pUC series and pBR322. Plasmid DNA may then, if desired, be amplified using any convenient wild-type host before isolation of the plasmid DNA and transformation back into the peptidase-deficient strain for screening.

The transformed peptidase-deficient bacteria are then screened for production of the desired Met-aminopeptidase by assaying crude extracts for their ability to cleave an appropriate substrate. A strain which shows an elevated level of Met-aminopeptidase is then conveniently used as a source for this enzyme or as a source of plasmid DNA for co-transformation with the expression vector for a recombinant protein.

In addition the plasmid DNA from this strain can be used to probe the genome of wild-type bacteria for the appropriate Met-amino-peptidase encoding sequences. Suitable stringencies are those characteristic of the hybridization conditions described specifically hereinbelow. These particular conditions need not be used, but those having the homology requirements. These retrieved sequences are also, of course, capable of being expressed, either under control of their own promoters, or using other promoters known in the art, such as the trp or penicillinase promoters.

The Met-aminopeptidase protein produced herein is purified and the purified protein used to obtain antibodies using suitable immunization protocols. The purification is conducted by disrupting the cells and isolating the enzyme from the supernatant lysate. This isolation may conveniently include treatment with an anion exchange resin under conditions wherein the protein is adsorbed, followed by elution in appropriate buffer. Suitable anion exchange resins include DEAE conjugated to various carbohydrate supports. Other anion exchange resins, well known in the art, may also be used.

Antibodies raised in response to the purified protein in rabbits, rats, or other mammals are useful in identifying Met-aminopeptidase proteins from a variety of microorganisms.

Large numbers of recombinant proteins are candidates for production as mature proteins in processed form, free of N-terminal methionine, using the method of the invention. For example, lymphokines such as IL-2, IL-1; the α- and γ-interferons (β-interferon normally contains an N-terminal methionine in its mature form); tumor necrosis factor; and other proteins associated with the lymphatic systems are thus produced. Also, various hormones, such as growth hormones, insulin, ACTH, endorphins, and other peptide hormones can be produced recombinantly. Other candidates for recombinant production include enzymes such as tissue plasminogen activator, urokinase, and enzymes useful in industrial applications such as alcohol dehydrogenase. Other proteins include toxins such as diphtheria and ricin toxin and various factors such as epidemal growth factor and transforming growth factor. The foregoing are merely exemplary, and, in principle, any desired protein, once its gene is obtained, can be expressed as a mature protein by linking it in reading frame to an immediately upstream ATG start codon and providing the necessary control sequences. The method of the invention permits any of these proteins to be conveniently produced without N-terminal methionine.

As stated above, the Met-aminopeptidase may be used in two general ways. The enzyme may be isolated, for example, from the specific cells producing it in relatively large quantity from plasmid-borne DNA, and added to an in vitro reaction mixture to effect N-terminal methionine processing, or the Met-aminopeptidase encoding plasmids may be cotransformed along with expression vectors for a desired protein into a recombinant bacterial host to permit in vivo processing.

In the in vitro approach, the purified Met-aminopeptidase is added to a reaction mixture containing unprocessed protein and suitable salt and buffer. The reaction mixture is incubated at a temperature of preferably about 30° C. overnight to permit the processing to occur. The enzyme exhibits a requirement for cobalt ion. A typical reaction mixture might contain about 1 mg/ml substrate protein, about 80 μg/ml of enzyme in pH 7-8 phosphate buffer and about 0.2 mM cobalt ion. The foregoing typical mixture is, of course, merely illustrative and the concentrations of the components can be varied in a continuum depending on the nature of the substrate and the time and temperature conditions employed. Complete processing can be verified by measuring the amount of methionine residue released, by comparing the mobility on SDS PAGE of the processed and unprocessed subject protein, or by sequencing the substrate material contained in the mixture.

In the in vivo approach, plasmid DNA is isolated from the Met-aminopeptidase source strain and used to transform cells which already harbor or which are concomitantly or subsequently transformed to harbor an expression system for the proteins. When used in bacterial sources for bacterial enzymes, the unprocessed substrates may be produced as a part of the complement of proteins ordinarily generated by the microorganism. More commonly, the peptidase generated in situ is used to process recombinantly produced proteins and the cells must be at some point transformed to contain the expression system for the desired protein.

C. Standard Techniques

Standard methods for transformation of bacteria, selection of successful transformants using markers, preparation of plasmid vectors, and screening of gene or cDNA banks are now well understood in the art. For convenience, a selection of procedures particularly useful in the example set forth below are presented here. Most such methods, or alternative workable ones, are found in Maniatis, et al, Molecular Cloning--A Laboratory Manual (1982), Cold Spring Harbor Press.

C. 1. Hosts and Control Sequences

The cells suitable for cotransformation with vectors bearing Met-aminopeptidase and expression systems for protein are bacterial. Most frequently the hosts are various strains of E. coli. However, other microbial strains may also be used, such as bacilli, for example Bacillus subtilis, various species of Pseudomonas, or other bacterial strains. In such procaryotic systems, plasmid vectors which contain replication sites and control sequences derived from a species compatible with the host are used. For example, E. coli is typically transformed using derivatives of pBR322, a plasmid derived from an E. coli species by Bolivar, et al, Gene (1977) 2:95. pBR322 contains genes for ampicillin and tetracycline resistance, and thus provides additional markers which can be either retained or destroyed in constructing the desired vector. Commonly used procaryotic control sequences which are defined herein to include promoters for transcription initiation, optionally with an operator, along with ribosome binding site sequences, include such commonly used promoters as the beta-lactamase (penicillinase) and lactose (lac) promoter systems (Chang, et al, Nature (1977) 198:1056) and the tryptophan (trp) promoter system (Goeddel, et al Nucleic Acids Res (1980) 8:4057) and the lambda derived P_(L) promoter and N-gene ribosome binding site (Shimatake, et al, Nature (1981) 292:128), which has been made useful as a portable control cassette, as set forth in copending application Ser. No. 578,133, filed Feb. 8, 1984, and assigned to the same assignee. However, any available promoter system compatible with procaryotes can be used.

C. 2. Transformations

The cells are transformed using calcium treatment employing calcium chloride, as described by Cohen, S. N., Proc Natl Acad Sci (USA) (1972) 69:2110, or the RbCl₂ method described in Maniatis, et al, (supra).

C. 3. Vector Construction

Construction of suitable vectors containing the desired coding and control sequences employs standard ligation and restriction techniques which are well understood in the art. Isolated plasmids, DNA sequences, or synthesized oligonucleotides are cleaved, tailored, and religated in the form desired.

Site specific DNA cleavage is performed by treating with the suitable restriction enzyme (or enzymes) under conditions which are generally understood in the art, and the particulars of which are specified by the manufacturer of these commercially available restriction enzymes. See, e.g., New England Biolabs, Product Catalog. In general, about 1 μg of plasmid or DNA sequence is cleaved by one unit of enzyme in about 20 μl of buffer solution; in the examples herein, typically, an excess of restriction enzyme is used to insure complete digestion of the DNA substrate. Incubation times of about one hour to two hours at about 37° C. are workable, although variations can be tolerated. After each incubation, protein is removed by extraction with phenol/chloroform, and may be followed by ether extraction, and the nucleic acid recovered from aqueous fractions by precipitation with ethanol followed by running over a Sephadex G-50 spin column. If desired, size separation of the cleaved fragments may be performed by polyacylamide gel or agarose gel electrophoresis using standard techniques. A general description of size separations is found in Methods in Enzymology (1980) 65:499-560.

Restriction cleaved fragments may be blunt ended by treating with the large fragment of E. coli DNA polymerase I (Klenow) in the presence of the four deoxynucleotide triphosphates (dNTPs) using incubation times of about 15 to 25 min at 20° to 25° C. in 50 mM Tris pH 7.6, 50 mM NaCl, 6 mM MgCl₂, 6 mM DTT and 5-10 μM dNTPs. The Klenow fragment fills in at 5' sticky ends but chews back protruding 3' single strands, even though the four dNTPs are present. If desired, selective repair can be performed by supplying only one of the, or selected, dNTPs within the limitations dictated by the nature of the sticky ends. After treatment with Klenow, the mixture is extracted with phenol/chloroform and ethanol precipitated followed by running over a Sephadex G-50 spin column. Treatment under appropriate conditions with Sl nuclease results in hydrolysis of any single-stranded portion.

Synthetic oligonucleotides are prepared by the triester method of Matteucci, et al (J Am Chem Soc (1981) 103:3185) or using commercially available automated oligonucleotide synthesizers. Kinasing of single strands prior to annealing or for labeling is achieved using an excess, e.g., approximately 10 units of polynucleotide kinase to 0.1 nmole substrate in the presence of 50 mM Tris, pH 7.6, 10 mM MgCl₂, 5 mM dithiothreitol, 1-2 mM ATP, 1.7 pmoles γ32P-ATP (2.9 mCi/mmole), 0.1 mM spermidine, 0.1 mM EDTA.

Ligations are performed in 15-30 μl volumes under the following standard conditions and temperatures: 20 mM Tris-Cl pH 7.5, 10 mM MgCl₂, 10 mM DTT, 33 μg/ml BSA, 10 mM-50 mM NaCl, and either 40 μM ATP, 0.01-0.02 (Weiss) units T4 DNA ligase at 14° C. (for "sticky end" ligation) or 1 mM ATP, 0.3-0.6 (Weiss) units T4 DNA ligase at 14° C. (for "blunt end" ligation). Intermolecular "sticky end" ligations are usually performed at 33-100 μg/ml total DNA concentrations (5-100 nM total end concentration). Intermolecular blunt end ligations (usually employing a 10-30 fold molar excess of linkers) are performed at 1 μM total ends concentration.

In vector construction employing "vector fragments", the vector fragment is commonly treated with bacterial alkaline phosphatase (BAP) in order to remove the 5' phosphate and prevent religation of the vector. BAP digestions are conducted at pH 8 in approximately 150 mM Tris, in the presence of Na⁺ and Mg⁺² using about 1 unit of BAP per μg of vector at 60° C. for about one hour. In order to recover the nucleic acid fragments, the preparation is extracted with phenol/chloroform and ethanol precipitated and desalted by application to a Sephadex G-50 spin column. Alternatively, religation can be prevented in vectors which have been double digested by additional restriction enzyme digestion of the unwanted fragments.

For portions of vectors derived from cDNA or genomic DNA which require sequence modifications, site specific primer directed mutagenesis is used according to the method of Zoller, M. J., et al, Nucleic Acids Res (1982) 10:6487-6500. This is conducted using a primer synthetic oligonucleotide complementary to a single stranded phage DNA to be mutagenized except for limited mismatching, representing the desired mutation. Briefly, the synthetic oligonucleotide is used as a primer to direct sysnthesis of a strand complementary to the phage, and the resulting double-stranded DNA is transformed into a phage-supporting host bacterium. Cultures of the transformed bacteria are plated in top agar, permitting plaque formation from single cells which harbor the phage.

Theoretically, 50% of the new plaques will contain the phage having, as a single strand, the mutated form; 50% will have the original sequence. The resulting plaques are hybridized with kinased synthetic primer at a temperature which permits hybridization of an exact match, but at which the mismatches with the original strand are sufficient to prevent hybridization. Plaques which hybridize with the probe are then picked, cultured, and the DNA recovered. Details of site specific mutation procedures are described below in specific examples.

C. 4. Verification of Construction

Correct ligations for plasmid construction are confirmed by first transforming E. coli strain MM294 obtained from E. coli Genetic Stock Center, CGSC #6135, or other suitable host with the ligation mixture. Successful transformants are selected by ampicillin, tetracycline or other antibiotic resistance or using other markers depending on the mode of plasmid construction, as is understood in the art. Plasmids from the transformants are then prepared according to the method of Clewell, D. B., et al, Proc Natl Acad Sci (USA) (1969) 62:1159, optionally following chloramphenicol amplification (Clewell, D. B., J Bacteriol (1972) 110:667). The isolated DNA is analyzed by restriction and/or sequenced by the dideoxy method of Sanger, F., et al, Proc Natl Acad Sci (USA) (1977) 74:5463 as further described by Messing, et al, Nucleic Acids Res (1981) 9:309, or by the method of Maxam, et al, Methods in Enzymology (1980) 65:499.

C. 5. Hosts Exemplified

Host strains used in cloning and expression herein are as follows:

For cloning and sequencing, and for expression of construction under control of most bacterial promoters, E. coli strain MM294 (supra), Talmadge, K., et al, Gene (1980) 12:235; Meselson, M., et al, Nature (1968) 217:1110, was used as the host. For expression under control of the P_(L) N_(RBS) promoter, E. coli strain K12 MC1000 lambda lysogen, N₇ N₅₃ cI857SusP₈₀, ATCC 39531 (hereinafter sometimes referred to as MC1000-39531) is used. To verify the presence of an insert, strains complementing the characteristic of the backbone vector in the region of the insert are used. For example, for the pUC series, E. coli strain DG99 , which produces blue colonies on X-gal indicator medium in the absence of insert and white colonies in the presence of insert is used.

D. Examples

The following examples are intended to illustrate, but not to limit, the invention.

D. 1. Preparation of a Source for Plasmid Met-Aminopeptidase Encoding DNA

Chromosomal DNA was extracted from E. coli Strain CM89 (supra) by the method of Silhavy, T. J., et al, Experiments with Gene Fusions (1984), Cold Spring Harbor Laboratory, New York, 137-139, and stored in 10 mM Tris, pH 8.0, 1 mM EDTA (TE) buffer. The DNA was digested with Sau3AI at 0.1 U/μg and 0.2 U/μg for 1 hr at 37° C. After reaction termination and ethanol precipitation, the partially digested DNAs were pooled and fractionated on a 10-40% sucrose gradient using a Beckmann SW28 rotor at 26,000 rpm for 24 hr at 15° C. Fractions containing 4-8 kb fragments were pooled, purified on a DE52 column, ethanol precipitated from the eluant and stored in TE buffer.

A 4 μg portion of the chromosomal DNA was then ligated with 0.5 μg of BamHI-digested, BAP-treated pUC18 vector fragments. The ligation mixture was used to transform E. coli DG99 and plated on lactose indicator plates to confirm the presence of inserts in the plasmids. Approximately 94% of the transformants indeed contained DNA inserts of approximately 4 kb.

Therefore, 18 μl of the ligation mixture was used to transform E. coli MM294 to Amp^(R) to obtain the gene library. Successful transformants were picked and used to inoculate 700 ml of ampicillin-containing culture medium for plasmid DNA preparation.

Plasmid DNA was obtained from the cells as described in C. 4 (Clewell, D. B., Proc Natl Acad Sci (USA) (1969)) above and used to transform E. coli CM89. Successful transformants selected for Amp^(R) were plated into microtiter trays for screening and approximately 1,000 colonies were screened.

Single colonies were picked into 200 μl minimal medium (see, for example, U.S. Pat. No. 4,518,584, incorporated herein by reference) supplemented 5% v/v each with 2X L-Broth (DIFCO) plus 1% NaCl, 10% Casamino acids, and 10X yeast nitrogen base. After overnight growth at 37° C., cells were washed twice in 0.1 M Tris.HCl pH 7.4. The cells were lysed by adding 20 μl of a 1 mg/ml lysozyme solution in the same buffer, followed by 3 cycles of freeze-thaw. Then 180 μl 0.1 M potassium phosphate buffer (KPO₄) pH 7.4+0.2 mM CoCl₂, containing 72 μg Met-Gly-Met-Met, 36 μg L-amino acid oxidase, 4-5 μg horseradish peroxidase, and 18 μg O-dianisidine dihydrochloride was added to each well; color formation was evident in Met-aminopeptidase-containing lysates. Of the approximately 1000 colonies screened, 10 showed increased rate of release of Met from Met-Gly-Met-Met, and were further screened for failure to release Leu from Leu-Gly-Gly under the same conditions. One successful colony was designated pSYC1174 . E. coli pSYC1174 was deposited with the American Type Culture Collection 27 August 1985 and has accession number 53245.

D. 2. Recovery of the Met-Aminopeptidase Coding Sequence

The E. coli strain harboring pSYC1174 (also designated 18E7) gave approximately one-hundred-fold higher activity in the Met-Gly-Met-Met substratemediated in vitro assay, conducted generally as described above.

Plasmid DNA was isolated from strain 18E7 and the Met-aminopeptidase-encoding plasmid pSYC1174 was isolated. pSYC1174 carried an insert of 3.2 kb in the pUC18 vector at the BamHI site. A 1.2 kb fragment was excised from the 5' end of the insert by EcoRI/ClaI digestion and inserted into pUC18 and pUC19 : pSYC1174 was digested with ClaI, blunted with Klenow and then with EcoRI to excise the 1.2 kb EcoRI/blunt fragment. This fragment was inserted into EcoRI/SmaI-digested pUC18 or pUC19 , which results in opposite orientation to the lac promoter on the host plasmids. Both of the resulting vectors pSYC1187 and pSYC1188 , when transfected into CM89, showed levels of amino peptidase production comparable to that shown by pSYC1174 . These results indicate that both the Met-aminopeptidase gene and its promoter are located within the 1.2 kb fragment.

FIG. 2 shows the complete nucleotide sequence of the 1.2 kb fragment determined by dideoxy sequencing. The open reading frame starting with ATG is at positions 219 -1010 and encodes a putative protein of 264 amino acids having a calculated molecular weight of 29,333. The first 64 amino acids deduced correspond to those obtained using amino acid sequencing of the purified protein as described in D. 3 below. The location of two tandem promoters associated with this open reading frame is also shown and are labeled P1 and P2. (The numbers in the lefthand column represent nucleotides, those in the righthand column, amino acids.)

The 1.2 kb fragment above was used to probe the genomic DNA of E. coli by Southern blot and hybridized to bands of 3.6, 5.2, and 1.35 kb that were generated by digestion with PstI, EcoRI, and BssHII, respectively, but not to other regions of the chromosome, thus indicating that the gene is present as a single copy in E. coli. Evaluation-of the sequence at the 3' end of the gene also leads to the conclusion that are probably no cotranscribed genes downstream from the Met-aminopeptidase. Evaluation of the sequence at the 5' end indicates the presence of tandem promoters, as shown.

The nucleotide sequence shown in FIG. 2 that encodes the deduced amino acid sequence 1-264 (or its complement) is generally useful to probe genomic microbial DNA for additional Met-aminopeptidase encoding sequences. Thus, this insert is a convenient tool to obtain this desired DNA from, for example, Bacillus subtilis, Pseudomonas, or yeast. Appropriate stringency is that associated with hybridization in a buffer containing 6×SSC, 0.01 M EDTA, 5×Denhardt's 0.5% SDS at 65° C. for sufficient time to complete reaction, followed by washing with 3× SSC at 65° C. That is, DNA which hybridizes to the above nick-translated probe under these conditions and which encodes protein having Met-aminopeptidase activity as herein defined, is included within the invention, and encodes Met-aminopeptidase protein within the invention.

D. 3. Characteristics of Met-Aminopeptidase from E. coli pSYC1174

The Met-aminopeptidase was purified from crude extracts of E. coli pSYC1174 and the purified protein was analyzed for ability to release amino acids using a variety of peptide substrates.

For purification, overnight cultures of E. coli pSYC1174 in Brain Heart Infusion broth were washed twice in KPO₄ buffer (20 mM, pH 7.4) with 0.2 mM CoCl₂, and sonicated. PMSF (0.1 mM) was added to the sonicate. The sonicate was centrifuged, and the supernatant passed over DEAE-Sepharose Fast Flow. The enzyme was eluted with a NaCl gradient, 0-0.25 M, in the same buffer. Fractions with Met-aminopeptidase activity were pooled, concentrated using 30,000 MW Centricon filters, and passed over an S-200 Sephacryl column, in the same buffer plus 1 mM methionine. Active fractions were pooled and concentrated as before.

The subunit molecular weight was determined to be approximately 32,000 by reducing SDS PAGE (Laemmli, J Mol Biol (1973) 80:575-599), and enzyme purity was estimated to be 95%. The protein is thus of approximately the predicted size for a 264 amino acid protein. Estimates of quantity based on densitometry of the stained gel showed the Met-aminopeptidase to be about 15-20% of cellular protein.

N-terminal sequencing was performed on the purified peptidase. Results for the first 65 amino acids are given in FIG. 1.

Purified Met-aminopeptidase was analyzed for the ability to release amino acids from a variety of peptide substrates, using a modified L-amino acid oxidase-HRP-ODAD assay, generally as described above and TLC method. For the first mentioned assay, 10 μl of the protein solution, in this case the lysate, was added to 90 μl of substrate solution containing 4 mM Met-Gly-Met-Met, 0.1 M potassium phosphate buffer, pH 7.5, and 0.2 mM CoCl₂. The tubes containing the reaction mixtures were incubated at 30° C. for 10 min and the reaction was stopped by placing the tubes in a boiling water bath for 2 min. After the addition of 0.9 ml color development mixture (0.1 M Tris HCl, pH 7.4, containing 0.2 mg L-amino acid oxidase, 0.03 mg horseradish peroxidase, and 0.2 mg O-dianisidine dihydrochloride), the tubes were incubated for 30-60 min at 30° C. and the optical density read at 440 nm.

The TLC method used silica gel plates with a solvent mixture of n-butanol-acetic acid-water (120:50:30 v/v). Amino acids released were detected and identified after spraying the plates with ninhydrin reagent.

Methionine was released from the following peptides:

Met-Ala-Met;

Met-Gly-Met-Met;

Met-Gly-Met;

Met-Ala-Ser;

Met-Gly-Gly;

Met-Ala-Pro-Thr-Ser-Ser-Ser-Thr-Lys-Lys-Thr-Gln-Leu; and

Met-Pro-Thr-Ser-Ser-Ser-Thr-Lys-Lys-Gln-Leu-Cys.

No amino acids were released from:

Met-Phe-Gly;

Met-Phe-Ala-Gly;

Met-Leu-Phe;

Met-Met-Met;

Leu-Leu-Leu;

Leu-Gly-Gly;

Met-Ala;

Leu-Gly;

Leu-Ala-Pro-Thr-Ser-Ser-Ser-Thr-Lys-Lys-Thr-Gln-Leu;

N-formyl-Met-Met-Met;

Met-Phe;

Met-Ser;

Val-Gly-Gly;

Thr-Gly-Gly;

Trp-Gly-Gly;

Phe-Gly-Gly;

Met-Glu-His-Phe-Arg-Try-Gly;

Met-Lys-Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg;

Gly-(Gly)₃ -Gly;

Phe-Gly-Gly;

Ser-Ser-Ser;

Pro-Thr-Ser-Ser-Ser-Thr-Lys-Lys-Gln-Leu-Cys;

Ala-Pro-Thr-Ser-Ser-Ser-Thr-Lys-Lys-Gln-Leu;

Thr-Val-Leu;

Arg-Gly-Gly;

Ala-(Ala)₂ -Ala;

Ala-Ala-Ala;

Glu-Gly-Phe;

Leu-Leu-Leu;

Tyr-Gly-Gly;

Ile-Gly-Gly;

Met-Arg-Phe acetate.

The peptidase activity was completely inhibited by 1 mM EDTA and requires about 1-200 mM phosphate ion and 0.02-2.0 mM Co⁺² for maximum activity.

Co⁺² could not be substituted for by Mn⁺², Cu⁺², Zn⁺², or Mg⁺² ions. The activity was also severely reduced when Tris was substituted for potassium phosphate buffer, but this activity can be restored by addition of sodium or potassium salts.

From the foregoing results, it is clear that the peptidase cleaves only N-terminal methionine and has other specificity requirements as well. The nature of the second and third amino acid in sequence appears to be significant, and a minimum of three amino acids in the peptide appears to be required. The results on short chain peptides may not, however, be completely determinative in terms of the requirements for succeeding amino acids, as folding in larger proteins may provide secondary and tertiary structures which alter the specificity as regards residues 2 and 3. In addition, the specificity is dependent on conditions, and peptides which fail to undergo hydrolysis under the specific conditions used may yet be hydrolyzed if the conditions are altered.

While in general Met-aminopeptidase cleaves N-terminal methionine when the second residue has a side chain with a radius of gyration smaller than 1.29 Å, and while it is generally expected that cleavage will not occur when the side-chain size is larger than 1.43 Å, exceptions do occur, notably that of the ricin A described below, wherein the radius of gyration for Ile, the second amino acid, is 1.56 Å. Thus, the effects of neighboring amino acids and the secondary and tertiary structure of the substrate also appear to have an impact on the specificity.

In addition, the Met-aminopeptidase purified from pSYC1174 as above was injected into rabbits using standard protocols and adjuvants, to obtain antisera specifically reactive with Met-aminopeptidase protein.

D. 4. Preparation of N-Terminal Met-less IL-2

E. coli MM294 carrying pSYC1143, an expression vector for a recombinant IL-2 mutein with the N-terminal sequence in the mature protein: Ala-Pro-Thr-Ser, under the control of the trp promoter was transformed with plasmid DNA (designated pSYC1174) prepared from E. coli pSYC1174 . Successful transformants were selected by Amp^(R) and Tet^(R), showing the presence of both desired plasmids.

Cells containing both plasmids were grown overnight at 37° C. in minimal medium plus tryptophan 50 mg/1, casamino acids 5 g/1 and both ampicillin 50 mg/1 and tetracycline 5 mg/1. The culture was then washed and resuspended in the same medium minus tryptophan, in order to de-repress IL-2 synthesis, and incubated at 37° C. for 4 hr.

The IL-2 thus produced is contained in intracellular refractile bodies. The refractile bodies were purified by repeated sonication and washing in 8 mM EDTA and finally resuspended in 5% SDS. The IL-2 was further purified by HPLC, using a vydac C4 reverse phase column and a gradient of water to acetonitrile in 0.1% trifluoroacetic acid. The N-terminal sequence was determined for the purified IL-2, and showed the following mixture:

    ______________________________________                                         Met-Ala-Pro-   0-5%                                                            Ala-Pro-       25-30%                                                          Pro-           70-75%                                                          ______________________________________                                    

Control cultures grown and induced as above, but containing only pSYC1143 gave produced IL-2 wherein the purified protein has the composition 70% Met-Ala-Pro and 30% Ala-Pro.

(pSYC1143 contains the IL-2 sequences under the control of the trp promoter and the cry positive retroregulator sequence. It is prepared from pFC51.T. which contains this expression system as a 0.95 kb EcoRI fragment, and pACYC184 , which is a host vector compatible with pUC18 carrying a Cm^(R) marker. The expression system is prepared as an EcoRI digest of pFC51.T and ligated into EcoRI-digested pACYC184 to obtain pSYC1143. pFC51.T is extensively described in U.S. Ser. No. 717,331, filed Mar. 29, 1985, assigned to the same assignee and incorporated herein by reference.)

D. 5. In Vitro Processing of Met-IL-2

The purified Met-aminopeptidase was also used to process the purified IL-2 in vitro. The reaction mixture contained 50 μl of stock solution containing 1.7 mg/ml of IL-2 prepared above in 0.05% SDS; 40 μl of 0.1 M potassium phosphate buffer, pH 7.5 containing 0.2 mM CoCl₂ ; and 10 μl of a 1:10 dilution of Met-aminopeptidase purified from E. coli pSYC1174 stock containing 8 mg/ml. The reaction mixture was incubated at 30° overnight and the degree of processing assessed using SDS PAGE and N-terminal sequencing. FIG. 3 shows the results of SDS PAGE performed on the unreacted IL-2 and IL-2 in the presence of enzyme. After incubation the presence of a slightly smaller molecular weight protein replaces the band shown by the original Met-preceded protein. The N-terminal sequence for the purified IL-2 before enzyme treatment was 74% Met-Ala-Pro-and 26% Ala-Pro; after enzyme treatment, it was 94% Ala-Pro- and 6% Met-Ala-Pro.

D. 6. Construction of Ricin A Expression Vectors

A host vector for the ricin A sequences, pSYC1089 , which contains the phoA promoter, leader and coding sequence with a modification to provide a NarI site at the C-terminal end of the leader sequence, followed by the B. thuringiensis positive retroregulator was constructed as follows.

pSYC997: PhoA promoter and Leader, Modified to Contain NarI Site

Plasmid pEG247, a 25 kb plasmid containing the 2.6 kb phoA structural gene as a HindIII/XhoI fragment was used as a source of the phoA gene. This plasmid was obtained from M. Casadaban and was constructed in a manner analogous to that set forth in Groisman, E. A., et al, Proc Natl Acad Sci (USA) (1984) 81:1840-1843. Indeed, by applying the procedures set forth in the foregoing reference, the phoA gene may be conveniently cloned into any desirable backbone vector.

The HindIII/XhoI 2.6 kb fragment from pEG247 was purified and cloned into pUC18 , a 2.7 kb plasmid containing an ampicillin resistance marker and a polylinker permitting convenient insertion of desired sequences. pUC18 was digested with HindIII/SalI, and the linear vector ligated with the isolated phoA fragment. The ligation mixture was used to transform E. coli DG99 , a strain comparable to E. coli JM103 or JM105 , to Amp^(R), and the construction of the intermediate plasmid pSYC991 in successful transformants, which had been screened for inserts into pUC18 , was verified.

pSYC997 which contains the desired NarI modification was prepared from pSYC991 by site-directed mutagenesis. The PvuII/PvuII 770 base pair fragment was obtained from pSYC991. It includes a portion of the phoA promoter and the upstream N-terminal sequences of the mature alkaline phosphatase, and thus, also, the entire leader sequence. This fragment was ligated into the SmaI site of M13 Mp11 and single stranded phage was prepared as template for the mutagenesis. In the mutagenesis, the synthetic 26-mer,

    5'-TTCTGGTGTCGGCGCCTTTGTCACAG-3'

(the superscript line shows the NarI site) was used as primer and probe. The mutagenized phage particles were then used to prepare RF-DNA as a source for the desired leader sequence containing the NarI site.

pSYC1015 : Cm^(R) Marker Backbone Vector

pSYC1015 , which provides chloramphenicol resistance, a replicon, and suitable restriction sites in the phoA gene, is also constructed from pSYC991. pSYC991 was first digested with HindIII/BamHI, and the approximately 2.6 kb fragment containing the phoA gene was purified and ligated with the purified 3.65 kb vector fragment from HindIII/BamHI-digested pACYC184. pACYC184 is available from ATCC and contains the chloramphenicol gene (Cm^(R)), a bacterial replicon, and HindIII and BamHI sites in the tetracycline resistance gene. The ligation mixture was used to transform E. coli MM294 to Cm^(R), and the construction of pSYC1015 was verified by restriction analysis and sequencing.

Additional phoA-Containing Intermediates

Two additional intermediate plasmids, pSYC1052 and pSYC1078 , were constructed in order to provide a suitable host vector for the B. thuringiensis positive retroregulator.

pSYC1052 was constructed by ligating the purified small HindIII/BssHII fragment containing the phoA promoter and NarI site from modified leader pSYC997 into HindIII/BssHII-digested pSYC1015 , which has, thus, the unmodified phoA sequences deleted. The resulting vector pSYC1052 was confirmed in E. coli transformants to Cm^(R).

pSYC1078 is a modified form of pSYC1052 with the BamHI site in front of the phoA promoter deleted. In order to delete this BamHI site, pSYC1052 was subjected to partial BamHI digestion, filled in using DNA polymerase I (Klenow) in the presence of the four dNTPs, and religated under blunt-end conditions. The desired resulting plasmid, now containing a unique BamHI site just 3' of the phoA gene, was confirmed after screening successful Cm^(R) transformants.

pHCW701 : Source of the Retrorequlator

The ability of the 3' sequences of the gene encoding crystal protein from B. thurinqiensis (the cry gene) to enhance the expression of upstream coding sequences was described and claimed in copending U.S. patent application No. 646,584, filed Aug. 31, 1984, assigned to the same assignee and incorporated herein by reference. Briefly, these sequences are characterized by a DNA sequence which transcribes to a corresponding RNA transcript capable of forming a stem and loop structure having a cytosine-guanine residue content of about 43%. When ligated about 30-300 nucleotides from the 3' end of the gene, a positive retroregulatory effect is shown on the gene expression. The positive retroregulator was prepared as a 400 bp EcoRI/BamHI restriction fragment, which was blunt-ended and ligated into pLW1 , an expression vector for interleukin-2.

(pLW1 is a pRBR322 derivative containing a replicon effective in E. coli, a Tet^(R) gene, the E. coli trp promoter, ribosome binding fragment and a 706 bp HindIII/PstI DNA fragment which includes the gene for human IL-2. pLW1 has been deposited with ATCC under the terms of the Budapest Treaty and has accession no. 39405.)

pHCW701 was completed by blunt-ending the 400 bp EcoRI/BamHI fragment containing the positive retroregulator of the cry gene with Klenow and the four dNTPs, and ligating the blunt-ended fragment using T4 ligase and ATP into StuI-digested plasmid pLW1 . Two possible orientations of insert can result, which are easily distinguishable by restriction analysis. The desired plasmid, designated pHCW701 , has the recreated BamHI site closer to the 3' end of the IL-2 gene. This plasmid was deposited with ATCC under the terms of the Budapest Treaty and has accession no. 39757.

Completion of pSYC1089

To complete pSYC1089 , pHCW701 was digested with EcoRI, filled in using Klenow and the four dNTPs, then digested with BamHI, and the 400 bp fragment containing the positive retroregulator recovered. pSYC1078 was digested with AvaI, filled in with Klenow and the four dNTPs, and then digested with BamHI. The ligation mixture was transformed into E. coli MM294 and the construction of the desired plasmid pSYC1089 , a 5.5 kb plasmid conferring Cm^(R), was confirmed. pSYC1089 contains the sequences for the phoA promoter and leader (with NarI site) sequence and structural gene immediately upstream of a BamHI site, followed by the positive retroregulator sequences of the cry gene.

Ricin A

The ricin A coding sequences were obtained from pRA123 , more specifically, an M13 subclone of pRA123 , described below, through construction of pRAT1 . pRA123 was deposited with ATCC Aug. 17, 1984 and has accession no. 39799. The construction of pRAT1 from pRA123 is described extensively in U.S. Ser. No. 653,515 filed Sept. 20, 1984, assigned to the herein assignee and incorporated herein by reference. Briefly, pRA123 , which contains the entire ricin A coding sequence (see FIG. 4), was modified to provide this sequence as a HindIII/BamHI cassette with a termination codon in the proper position after amino acid 265, and with a start codon in position immediately preceding the mature sequence. pRA123 was digested with BamHI and the approximately 896 bp BamHI/BamHI fragment was isolated and subcloned into M13 mp18 in an anti-sense orientation relative to the lac promoter in the M13 vector. The phage single stranded DNA was subjected to two stages of primer directed mutagenesis using as primers, the sequence:

    5'-CACAGTTTTAATTGCTTATAAGG-3',

which places the TAA termination codon in proper reading frame at the terminus of the ser-gln-phe C-terminus of the ricin A chain followed by:

    5'-CTTTCACATTAGAGAAGCTTATGATATTCCCCAAAC-3',

which places the desired HindIII/ATG start codon diad immediately upstream of the N-terminal ile-phe-pro-lys sequence of ricin A. The modified phage were identified after each mutagenesis using the appropriate above primers as probes. The desired constructs were then double digested with HindIII and BamHI and the appropriate ricin A coding fragment isolated. pRAT1 was completed by ligation of the HindIII/BamHI fragment with HindIII/BamHI digested pTRP3 . pTRP3 is a pBR322 based vector containing the trp promoter with a downstream HindIII site; pTRP3 was deposited with ATCC Dec. 18, 1984, and has accession no. 39946.

pRAP229 was constructed using coding sequences derived entirely from pRAT1 . It was obtained by a three-way ligation of (1) the large NarI/BamHI repliconcontaining fragment of pSYC1089 which provides, in order, B. thuringiensis-positive retroregulator sequences, the chloramphenicol resistance marker, a compatible replicon, and the phoA promoter and leader sequences; (2) ClaI/BamHI-digested pRAT1 which provides a 500 bp fragment encoding the C-terminal portion of ricin A properly terminated; and (3) the N-terminal sequence as provided by an approximately 350 bp ClaI/ClaI fragment from pRAT1. It is clear that the ricin A sequences could also have been, and might preferably be, prepared as a ClaI(partial)/BamHI-excised fragment from pRAT1 .

The resulting fusion (1) retains the start codon of the ricin A chain preceding the isoleucine residue; due; (2) is separated by 7 bp and thus out of reading frame relative to the leader sequence; (3) extends the phoA leader by the tripeptide Ile-Ser-Leu; and (4) allows for termination of the leader sequence at a TGA codon out of frame with, but proximal to, the start codon of ricin A. pRAP229 was deposited at ATCC on March 8, 1985 and has accession no. 53408.

D. 7. Production and Processing of Ricin A in E. coli

E. coli MM294 was transformed either with pRAP229 alone or cotransformed with pRAP229 and pSYC1174 . The transformed cultures were grown under conditions similar to those described by Michaelis, et al, J Bact (1983) 154:356-365. The cells were induced by lowering the exogenous phosphate concentration and maintaining the cultures for 16-17 hr.

The cells were harvested, and whole cell extracts prepared by sonication in the absence of detergent were assayed for expression using Western blot employing rabbit antisera to native ricin A. To assess the N-terminal methylation, the ricin A was purified and subjected to Edman degradation for quantitative analysis. When the cells were transformed with pRAP229 alone, 40% of the ricin A contained an N-terminal methionine; the cotransformed cells produced ricin A having only 9% of the chains methylated.

D. 8. In Vitro Processing of Met-Ricin A

Purified ricin A (600 μg) from the pRAP229 transformed cell lysates was mixed with 24 μg of Met-aminopeptidase purified from E. coli pSYC1174 in 0.2 mM cobalt chloride and 0.1 mM potassium phosphate buffer, pH 7.5, having 0.3 ml total volume. The reaction mixtures were incubated overnight at 30° C. and stopped by heating in a boiling water bath. The N-terminal sequence was determined by Edman degradation after desalting the reaction mixture. Control mixtures containing no Met-aminopeptidase showed 40% of the ricin A with N-terminal methionine, the same fraction normally found in the lysate, as described above. Reaction mixtures containing the Met-aminopeptidase enzyme contained ricin A wherein only 8% of the protein contained N-terminal methionine, substantially the same fraction as obtained in pRAP229 /pSYC1174 transformed cells.

On Aug. 27, 1985, E. coli strain pSYC1174 was deposited with the American Type Culture Collection (ATCC 53245) under the terms of the Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purposes of Patent Procedure and the Regulations thereunder (Budapest Treaty). This assures maintenance of a viable culture for 30 years from date of deposit. The organisms will be made available by ATCC under the terms of the Budapest Treaty, and subject to an agreement between Applicants and ATCC which assures unrestricted availability upon issuance of the pertinent U.S. patent. Availability of the deposited strains is not to be construed as a license to practice the invention in contravention of the rights granted under the authority of any government in accordance with its patent laws.

The foregoing written specification is considered to be sufficient to enable one skilled in the art to practice the invention. The deposit of materials herein does not constitute an admission that the written description herein contained is inadequate to enable the practice of any aspect of the invention, including the best mode thereof, nor is this deposit to be construed as limiting the scope of the claims to the specific illustrations which materials deposited represent. 

We claim:
 1. A method of preparing N-terminal methionine-free protein which comprises treating a Met-preceded protein with an effective amount of Met-aminopeptidase, wherein said Met-aminopeptidase is prepared byculturing transformant cells; which cells have been transformed with a recombinant DNA expression system comprising control sequences operably linked to DNA encoding Met-aminopeptidase, which DNA encoding Met-aminopeptidase hybridizes to a labeled DNA comprising the DNA of FIG. 2 encoding amino acids 1-264 of that figure or its complement, under hybridization conditions that are equivalent in stringency to hybridization in a buffer containing 6× SSC, 0.01 M EDTA, 5× Denhardt's, 0.5% SDS at 65° C. for sufficient time to complete reaction, followed by washing with 3× SSC at 65° C.; said culturing under conditions which favor expression of said DNA encoding Met-aminopeptidase; and recovering Met-aminopeptidase from the transformants.
 2. The method of claim 1 wherein the Met-aminopeptidase is derived from E. coli, and the Met-preceded protein is IL-2.
 3. The method of claim 2 wherein the E. coli is strain pSYC1174 .
 4. The method of claim 1 wherein the Met-aminopeptidase is derived from E. coli, and the Met-preceded protein is ricin A.
 5. The method of claim 1 wherein the DNA encoding Met-aminopeptidase is prepared by a process comprising screening a microbial genomic library with a labeled DNA probe comprising the DNA of FIG. 2 encoding amino acids 1-264 of that figure or its complement, and recovering the DNA which hybridizes to said probe.
 6. The method of claim 1 wherein the DNA encoding Met-aminopeptidase is prepared by a process comprising:preparing a plasmid borne gene library from a first bacterial strain; transforming said gene library into a second bacterial strain that is deficient in peptidase activity; screening transformants for Met-aminopeptidase activity; and recovering plasmid DNA from transformants having said activity.
 7. The method of claim 1 wherein the DNA encoding Met-aminopeptidase is obtained by a process comprising:screening bacterial transformants for Met-aminopeptidase activity and recovering plasmid DNA from transformants having such activity; wherein the transformants were prepared by preparing a plasmid borne gene library from a first bacterial strain; and transforming said gene library into a second bacterial strain that is deficient in peptidase activity. 